Implement <prefix>/server/health

This indicates if the Sia server is available and connected to the MQTT
broker.

Using the MQTT Will messages these will also be sent if the Sia server
unexpectedly crashes.

Using the retained flag it is ensured that freshly connected MQTT
clients will receive these messages even when they missed the initial
one from the Sia server.

1 files changed, 11 insertions, 0 deletions

diff --git a/mqtt.go b/mqtt.go
index 5c1b1ad..6d01103 100644
--- a/mqtt.go
+++ b/mqtt.go
@@ -1,6 +1,7 @@
 package main
 
 import (
+	"fmt"
 	"log"
 	"time"
 
@@ -18,11 +19,19 @@ const (
 	MQTT_KEEPALIVE_PERIOD      = 2 * time.Second
 )
 
+var (
+	mqttServerHealthTopic string
+)
+
 type MQTTMessage struct {
 	Topic   string
 	Payload []byte
 }
 
+func init() {
+	mqttServerHealthTopic = fmt.Sprintf("%s/server/health", TOPIC_PREFIX)
+}
+
 func MQTTRun(tx chan MQTTMessage) {
 	opts := mqtt.NewClientOptions()
 	opts.AddBroker(BROKER)
@@ -34,6 +43,7 @@ func MQTTRun(tx chan MQTTMessage) {
 	opts.SetConnectRetry(true)
 	opts.SetConnectTimeout(MQTT_CONNECT_TIMEOUT)
 	opts.SetKeepAlive(MQTT_KEEPALIVE_PERIOD)
+	opts.SetWill(mqttServerHealthTopic, `bad`, QOS, true)
 
 	client := mqtt.NewClient(opts)
 
@@ -52,6 +62,7 @@ func MQTTRun(tx chan MQTTMessage) {
 
 func MQTTOnConnectHandler(c mqtt.Client) {
 	log.Printf("Connected to MQTT broker (%s)", BROKER)
+	c.Publish(mqttServerHealthTopic, QOS, true, []byte(`good`))
 }
 
 func MQTTConnectionLostHandler(c mqtt.Client, err error) {